A printed circuit board (PCB) is a support for electronic components. PCBs are used for mounting items mechanically and for making electrical connections. Virtually every electronic device comprises one or more printed circuit boards. Printed circuit boards are made of electrically insulating material having attached conductive materials called conductor paths. A common insulating material is fiber-reinforced plastic. The conductors are usually etched from a thin layer of copper.
Examples of electronic devices are electron tubes, semiconductor diodes, transistors, integrated circuits, resistors, capacitors and inductors. An integrated circuit (IC) is an electronic circuit housed on one single (semiconductor) substrate circuit consisting of electronic devices that are interconnected together. The main feature of integrated circuits is a large number of different types or same active and passive components and connecting conductive paths on or in a single-crystal substrate. A distinction is made based on the type of signal as digital, analog or mixed ICs. Typical ICs are data memory storage devices, processors, logic ICs, ASICs, DSPs, microcontrollers, D/A and A/D converters and others.
Most examples of ICs comprise the same starting material. Silicon is used as the substrate material in the majority of instances (gallium arsenide is used in special situations). Silicon is an elemental semiconductor. By using suitable dopants such as boron or arsenic, certain properties in silicon—especially conductivity—can be altered, and most importantly, can be amplified. Silicon's properties, however, are highly dependent on temperature. At low temperature, the semiconductor crystal acts as an isolator, i.e. it has no movable charge carriers. As the temperature increases, the number of open charge carriers increases, which in turn increases conductivity. Manufacturers of electronic components often specify −40° C. as the lowest limit for functionality.
In process automation technology, particularly when automating chemical or process engineering processes and/or when automating processes in order to control industrial systems, process-oriented installed measuring devices and field devices are used. Configured field devices can act as sensors to monitor process metrics such as pressure, temperature, flow, level or measures of measurement variables for fluid and/or gas analysis such as pH, conductivity, concentrations of certain ions, chemical compounds and/or concentrations or partial pressures of gases.
Frequently, a plurality of different types of sensors are used in a process installation. A sensor installed at a particular installation location, e.g. an installed sensor configured at a particular installation location for detecting one or more measurement variables, may be combined with a transmitter to function as a measuring point.
A sensor generally has a transmitter which is adapted to detect the measured variable to be monitored and to generate an electric measurement signal correlated to the measurement variable's current value. In order to further process the measurement signal, an electronic circuit is used that is configured to further prepare the electrical measurement signal, e.g. to digitize it, to convert it into a measurement value for the measurement variable and/or into a variable derived from the measurement value, and to optionally output it to a superordinate unit. The circuit may include other functions in addition to creating and/or forwarding a measurement value. The circuit can, for example, be adapted to carry out additional evaluations on the measurement values or to run a sensor diagnosis, during which the sensor's current state is detected, and a prediction is made regarding the sensor's remaining service life. The circuit may be arranged wholly or partially in the transmitter.
Field devices are required in various industries and applications. In extracting oil and gas, the US, Canada and Russia are the main producing countries. Here, the oil and gas reserves are often in very cold areas where temperatures can often fall below −50° C. As already mentioned, under certain conditions, electrical circuits sometimes fail at these temperatures.
One conventional way of dealing with low temperatures is to wrap field devices that have to be used at very low outdoor temperatures in insulating material and heat them from the outside. The disadvantage of doing this is that an additional power supply is required to supply this heat, which can necessitate a lot of added time and expense. In addition, the field device loses its compact design. Also, the insulation material might cover or hide the control or signaling units.
Applying heat to a printed circuit board also ends up heating it very unevenly. Furthermore, some components on the circuit board generate heat themselves (e.g. processors). Heat distribution is thus irregular, giving rise to “hot spots”, i.e. points that become very warm—even hot—while some spots remain cold. These temperature gradients can subject the components to mechanical stress, and in extreme cases, can cause the components to detach from the printed circuit board.
DE 10 2005 062 421 A1 contains a description for a heater for a field device display module, wherein the heater comprises a heating element. The heating element is adapted to the shape of the field-device display module and is configured to convert electric current into heat energy. The heating element can be coupled to the field device display module to keep it warm.
DE 10 2013 108 531 describes a field device with a heating element and a control. It includes a temperature measuring unit that determines the temperature in the vicinity of a temperature-sensitive component, and the control activates the heater so that the ambient temperature for temperature-sensitive component remains above a predetermined threshold. Since the heater control itself mainly consists of electronic components, options are limited for starting the heating element by setting it to start when temperatures fall below the operating temperature.
EP 3 371 645 shows a heater that is embedded in a printed circuit board to melt solder on the board. The heater is made of copper with a layer of magnetically permeable material. A high-frequency current is constantly applied to the heater. US 2006/0065431 shows a similar structure, where again, a constant current is applied to a heater on a printed circuit board in order to melt solder in order to create an electrical contact for components without having to use a reflow oven.
Published US Application 2013/0180973 shows a circuit board having a top conductive layer and a lower conductive layer. A plurality of electronic components is mounted on at least one of the layers. A heating layer consisting of horizontal and vertical heating structures is provided between the two conductive layers to generate heat and to transport the electronic component. A current is applied to the heating structures, which in turn generates heat. The heating structure is a conductive copper layer.